The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 172

With regards to solar output, during the period 1645-1715 solar activity - as
inferred by sunspot activity - was low, with some years having no sunspots at all. This
period is known as the Maunder Minimum after the astronomer E. W. Maunder who
noticed, from records, that there were few sunspots during that period. For example,
during one 30-year period within the Maunder Minimum astronomers observed only
about 50 sunspots, as opposed to a more typical 40 000-50 000. What the precise
link is between low sunspot activity and cooling temperatures has not been firmly
established, but the coincidence of the Maunder Minimum with the deepest trough
of the Little Ice Age is highly suggestive.
One might suspect that if solar output, hence Earth's insolation, during the Little
Ice Age was less, this would have a uniformly global effect. This is not necessarily
so, given (as noted in Chapter 1) that the northern hemisphere is predominantly land
and the southern ocean and that these react differently to incoming energy. Second, in
terms of biology, reduced insolation throughout the year may only affect part (and/or
the length) of the thermal growing season. This may explain why Himalayan tree-ring
series show little evidence of the Little Ice Age as these dendrochronological series
are mostly affected by pre-monsoon temperatures and rainfall (Boragaonkar et al.,
2002).
In addition, there may have been changes in atmospheric and/or ocean circulation
that may have exacerbated any global change in some regions and ameliorated them in
others. In 2006 David Lund, Jean Lynch-Steiglitz and William Curry reported on their
use of Foraminifera in the Florida Straits. Their work suggests (there are some reason-
able working assumptions) that during the Little Ice Age (from around 1200 to 1850)
the Gulf Stream's normal warm current flow of 31 Sverdrups (Sv) 8 was reduced by
10%. This would have reduced heat flow to the Atlantic off western Europe and Green-
land, so is a likely explanation for why the Little Ice Age was so pronounced there.
Of course, in terms of nomenclature, the Little Ice Age was neither an ice age
nor a glacial. That such a palaeoclimatologically powerful term has come to describe
what was a short and modest shift in climate is simply due to it being the major
climatic event taking place within modern recorded history. In terms of psychology
and the social science dimensions of climate change, the use of the term Little Ice Age
demonstrates that hemispheric and global climate change is hugely relevant to human
society. We will return to the biological and human ecological effects of Holocene
climate change shortly, but for now it might be interesting to note that our cultural
heritage of a Dickensian white Christmas owes itself to the Little Ice Age.
The recovery from the end of the Little Ice Age is well documented by over
a century of direct observation in Europe and North America. Globally, snowline
retreat since 1860 to the end of the 20th century suggests a global warming of 0.6-
0.7 ◦ C. And between 1880 and 1930 records at Oxford, England, show that there was
a 10% increase in the thermal growing season.
Notwithstanding shorter-term events such as the warm MCA and the Little Ice Age,
similar to the long-term study of Mann et al. (2009) using climate proxies, Darrell

8

1Sv = 10 6 m 3 s − 1 . The Sverdrup is a unit of measure of volume of water transport used exclusively in
oceanography. It is not an SI unit, and its symbol is the same as that for the sievert, Sv (a unit referring
to radiological dose of ionising radiation).